Covers would be better since users would not be tempted to ask "What does this button do?" I'm thinking of a situation where you are training a user to use an application. First you take them through the basic features. Then the intermediate features. And then the advanced features. It would help for them to not have distractions as they progress through each stage.

sweet idea man! Ok now jimmy push the next button...uh excuse me mrs. bing bong...yes jimmy?...uh wheres the next button...ok who put white out all over the screen?!...its not white out, its WINCOVER!!!...Sweet gimmie!...it hasn't been made yet...aw poo.

It's more like, "OK, lets add a 10 Ohm resistor to the string and see how that effects things". Meanwhile half your class of 12 students are distracted with all sorts of other things such as personalizing their desktop background, screensavers, email etc, etc.